The present invention relates to a manufacturing method by which to manufacture a thin glass plate (sheet glass) such as a glass substrate for use with liquid crystal, as well as a glass plate manufacturing apparatus and a liquid crystal device.
As manufacturing methods for this type of sheet glass, the float method and the downdraw method are known, but when it is desired to manufacture a glass substrate for a liguid crystal display, the downdraw method is more widely used for cost reasons, as it entails little or no polishing after molding.
An example of the downdraw method ls described in connection with the method disclosed in Japanese Patent. Application Laid-Open No. 10-291826. The method disclosed therein involves the manufacturing a glass substrate by continuously providing molten glass from a melting tank along molding surfaces, fusing the glass on both sides below the molds, and drawing the glass downward using rollers at the edge areas of the glass.
Because the glass substrate obtaied through this type of manufacturing method is formed such that both surfaces at the time of molding are free surfaces. and such that the glass surfaces that were in contact with the mold are fused together, it offers superior smoothness and flatness.
Generally, the glass plate that is obtained through this manufacturing method and has outer dlmonsions of 1mxc3x971 m and a thickness of 0.7 mm is cut to a size of 550xc3x97650 nm or 600xc3x97720 nm, and is used as a glass substrate for a liquid crystal display. Examples of such a liquid crystal device glass substrate include a TFT glass substrate or a color filter glass substrate.
For a TFT glass substrate. a thin film transistor (TFT) is formed on the surface of the glass substrate, while for a color filter glass substrate, a color filter is formed on the surface of the glass substrate. A liquid crystal device is formed by encasing liquid crystal between two glas substrates an which these thin film are formed.
However, glass plates manufactured using the conventional downdraw method. have the problem that a substantial amount of strain occurs in the widthwise direction (the direction perpendicular to the direction in which the glass is drawn). This strain is caused by a plate thicunes distribution peculiar to the downdraw method. The plate thickess of sheet glass manufactured using the dowzdraw method is larger at the edge areas in the widthwise direction than at the center area (called the xe2x80x98surface areaxe2x80x99 below). Consequently, when the sheet glass is slowly cooled from a high temperature after molding. because the cooling speed of the surface area is faster than the cooling speed at the edge areas, compressive stress occurs at the edge areas, while tensile stress occurs at the surface area, and as a result, mlinute strain occurs in the vidthwise direction. In addition, this strain tends to be relatively larger at the edge areas than at the surface area.
When a piece of this sheet glass having this type of strain distribution is cut out in the sie appropriate for a display apparatus glass substrate. the strain distribution continues to exist in a reconstructed state, which causes minute deformation of the glass substrate.
When a thin film transistor (TFT) or a tain film pattern for a color filter is formed via photolithography on the surface of the glass substrate having this type of minute deformation, exposure is not perfored accurately during the photolithography process, and as a result, the problem occurs that the precision of the thin film pattern is reduced.
Furthermore, when a TFT glass substrate is combined with a color filter glass substrate, the alignment mmrks become offset due to the minute deformation in the glass plates, and as a result, the problem occurs that the yield of the liquid crystal devices dealings.
In particular, when the glass substrate is large in size, the problem occurs that the amount of defd atlon of the glass substrate is accordingly large and the amount of offset of the pattern positioning is large as well.
The present invention was created in view of the above problems, and has the objects described below.
One object of the present invention is to inhibit the occurrence of minute strain or minute deforzation in the downdraw method.
Another object of the present invention is to prevent the occurrence of positioning offset of the pattern formed through photolithography on the surface of a glass substrate used in a display apparatus.
Yet another object of the present invention is to prevent the occurrence of strain when a piece of shoet glass that is formed through the dommdraw method is cut into a prescribed size.
Still another object of the present invention is to improve the yield obtained during manufacture of liquiid crystal devioes.
Accordingly, the present invention proposes a manufacturing method wherein, in order to ellimnate stratn caused by temperature variations in the vidthwise diroation of the sheet glass when the downdrav method is used, carries out strain reduction processes during the slow cooling of the sheet glass.
Construction 1: A glass plate manufacturing method for manufacturing a glass plate by forming molten glass into a sheet using the downdraw method and subjecting the obtained sheet glass to slow cooling using a heat treating unit, wherein a strain reduction process to reduce strain occurring in the sheet glass due to the temperature difference between the edge areas and the surface area in the widthwlse direction of the sheet glass is performed during the slow cooling.
For a glass material suitable for the downdraw method, it is preferred from the standpoint of maintaining transparency that the material have a liquid phase temperature of 1200xc2x0 C. or lower, and ideally a liquid phase temperature of 1100xc2x0 C. or lower. It is further preferred that the thickness of the sheet glass be between 0.5 and 1.0 nm.
It is also preferred that the strain reduction processes be carried out mmedilately after molding.
Construction 2: The glass plate manufacturing method according to the Construction 1, wherein the temperature difference occurs between the center region of the surface area and the edge areas.
Construction 3: The glass plate manufacturing method according to the Construction 1 or the Construction 2, wherein the sheet glass subjected to slow cooling exhibits a widthvise terature distribution such that the surface area has a lower temperature than the edge areas.
Construction 4: The glass plate manufacturing method according to any of Constructions 1 through 3, wherein the strain reduction processes are carried out based on the strain distribution in the post-molding glass plate measured in advance using the optical heterodyne method.
Because the strain addressed by the present invention comprises minute strain (deformation) it is preferred that the birefringence amount be measured through optical heterodyne measurement, which can measure minute strain (minute deformation) with high precision, and that the strain reduction processes be carried out based on the results of this measurement. Measurement of the atrain using the optical heterodyne method allows the widthwise temperature distribution in the sheet glass after molding to be easily understood.
Construction 5: The glass plate manufacturing method according to any of Constructions 1 through 4, wherein the strain reduction process comprises a heat treatment that forms a prescribed temperature distribution in the widthwise direction of the sheet glass so that the temperature difference in the widthwise direction of the sheet glass after molding is reduced when the sheet glass is subjected to slow cooling by means of the heat treating unit after molding.
It is preferred that the temperature range used for slow cooling of the sheet glass be 600 to 850xc2x0 C. It is further preferred that the method for reducing the temperature difference in the vidthwise direction of the sheet glass comprise heat treatment by a heat treating unit having a temperature distribution whereby the widthwise temperature distribution of the sheet glass is offset.
Because this heat treatment is carried out during the slow cooling process. the vidthwise temperature distribution of the sheet glass changes with the cooling tie. Therefore. it is preferred that the temperature distribution of the heat treating unit change continuously or in steps along the direation of drawing in accordance with the temerature distribution of the sheet glass.
Construction 6: The glass plate manufacturing method according to Construction 5, wherein the thermal process is carried out at least through a process in which the sheet glass is slowly cooled from the molding temperature to a temperature near the strain point.
It is preferred that the heat treatment be carried out within a temperature range from the moldigg te pesature of the sheet glass to an extraction temperature equal to or lower than the strain point, but it is effective if the heat treatment is carried out through the slow cooling process in which the sheet glass is slowly cooled from the molding temperature to a temperature near the strain point. It is preferred that the thexIal process be carried out through a process in whioh the sheet glass ls slowly cooled from a temperature that is 400 to 500xc2x0 C. lower than the molng temperature to a temperature near the strain point. In addition, it in preferred that where the widthviue temperature of the sheet glass is made essentially uniform at a temperature near the strain point, and slow cooling at or below the strain point is thereafter carried out, that the uniform temperature state of the sheet glass be maintained and that slow cooling be further carried out.
Furtherore, the preferred range for the tempeature near the strain point is 50xc2x0 C. above and below the strain point.
Construction 7: The sheet glass manufacturing method according to Construction 5 or Construction 6, wherein the heat treatment is a process in whiah the temperature distribution that is formed, in the widthwise direction of the sheet glass, by the heat treating unit for beating the sheet glass is set such that the temperature difference in the the widthwise direction of the sheet glass may be reduced.
Where the temperature distribution is formed via the heat treating unit, it to preferred that, rather than foming such temperature distribution suddenly near the strain point, the temperature distribution near the strain point be used as a reference, and that a temperature distribution obtained by flattening this temperature distribution near the strain point be formed in steps or continuously in the slow cooling process from the strain point and the molding temperature.
Construction 8: The glass plate manufacturing method according to any of Constructions 1 through 3, wherein the strain reduction process reduces strain oocurring from the surface area to the edge areas by increasing the amount of extension of the edge areas relative to the amount of extension of the surfaoe area in acoordance with the heat contraction difference occurring between the edge areas and a the surface area in the wldthwise direction of the sheet glass.
It is preferred that the strain reduction processes be carried out through slow cooling within a temperature rage of 600 to 850xc2x0 C.
Construction 9: A glass plate manufacturing meobod wherein the maximum strain of the manufactured sheet glass is 0.07 kg/mm2 or less.
It is further preferred that the maxlmum strain be 0.04 kg/mm2or less.
Construotion 10: The glass plate manufacturing method according to any of Constructions 1 through 9, wherein the glass plate is a glass substrate used in a display apparatus.
It is preferred that the display apparatus comprlse a liquid crystal device, and that the glass substrate for the liquid crystal device have an expansion coefficient of 32-38xc3x9710xe2x88x927/xc2x0 C., and that the strain point be at least 650xc2x0 C.
It is preferred that the glass have a composition comprising 60-70% SiO2, 7-12% B2O2, 9-13% AL2O3, 1-8% MgO, 2-8% CaO, 0.5-5% SrO, and 0.5-5% BaO, where the percentages are expressed in term of molar percentages.
It is also preferred that the glass have a composition comprising 65-75% SiO2, 6-11% B2O3, 8-15% AL2O3, 3-15% MgO, 0-8% CaO, 0-1% SrO, and 0-1% BaO.
Construction 11: A glass plate manufacturing apparatus that includes a molding unit that molds molten glass provided continuouwsly from a molten glass storage tank into a sheet;
a drawing unit that draws down the sheet glass molded by the molding unit and existing in a softtned state; and
a strain reduction unit that reduces the strain caused by the temperature difference occurring from the edge areas to the surface area in the widthwlse direction of the shoet glass.
Construction 12: The glass plate manufacturing apparatus according to Construction 11, wherein the strain reduction unit ls a heat treating unit that slowly cools the sheet glass after molding, and sets in the widthwise direction of the sheet glass a temperature distribution that reduces the temperature difference in the widthvise direction of the post-molding sheet glass.
It is preferred that the beat treating unit be located on one side or both sides of the sheet glass. It is also preferred that the heat treating unit be located near the surface or surfaces of the sheet glass.
Construction 13: The glass plate manufacturing apparatus according to Construction 11, wherein the strain reduction unit has a heat treating unit that slowly cools the sheet glass after molding and a unit that performs control to ensure that the amount of extension of the edge areas is larger than the amount of extension of the surface area in accordance with the tewpcrature difference from the surface area to the edge areas of the sheet glass that is being slowly cooled by the heat treating unit.
Construction 14: A liquid crystal device comprising, liquid crystal held between a pair of glass plates formed through the glass plate manufacturing method accordng to any of Constructions 1 through 10.